1. Field of the Invention
The present invention relates to a visible and infrared light image-taking optical system, and more particularly, to a visible and infrared light image-taking optical system which enables the same object to be taken simultaneously using visible light and infrared light.
2. Description of the Related Art
Japanese Patent Application Laid-Open No. 2005-4181 and No. 2004-45266 have proposed a lens system which takes the same object image using visible light and infrared light through a lens device used in a TV camera for broadcasting or for business use. These lens systems permit object light incident on an image-taking lens to be separated into object light in a visible light region and object light in an infrared light region by a color separation prism, so that an image by the each object light is formed at a visible light image pickup element and an infrared light image pickup element, respectively. The lens system according to Patent Unexamined Patent Application Publication No. 2005-4181 includes a correction lens for correcting an image formation position of object light in an infrared light region and adjusting a position of the correction lens corrects axial chromatic aberrations of infrared light.
As one of video image synthesis methods used for synthesis of a live-action video image and a video image prepared by a computer or the like, the chromakey method which replaces a video image in a specific color region of a taken video image with another video image is generally used. In synthesizing video images using the chromakey method, for example, for production of a program for broadcasting, a screen of a mono color such as blue or green is disposed at a background portion of an object such as a performer (a main object) to be captured as a video image. An image region of the screen in a taken video image is identified by color and a key signal for distinguishing a main-object region from a background region is created. A synthesized video image is created by replacing the video image in the background region with another video image created by a computer or the like by use of the key signal.
On the other hand, such a chromakey method requires to keep the screen in a fixed range of brightness, chroma and hue, so that flexibility of setting illumination for the main object is constrained. Moreover, there are some problems such as the constraint that clothes or an accessory of the main object cannot use the same color as a screen of the background.
Accordingly, as a method of video image synthesis for solving such problems, there has been proposed a method for creating a key signal using an infrared video image taken by a camera for infrared light. This permits the screen formed of recursive reflection material having the property of reflecting incident light strongly toward a light source to be disposed at a background portion of the main object. Infrared light is irradiated from the camera for infrared light toward the main object and an infrared light video image is taken by the infrared light camera. At this time, the infrared light is diffused on a surface of the main object and, on the screen of the background, the infrared light is reflected strongly toward the light source, thus achieving an infrared video image with a dark main object and a bright background portion. Detecting a difference (contrast) in the brightness creates a key signal for distinguishing a main-object region from a background region. The infrared light camera is disposed so as to take an infrared video image having the same view angle and the same object distance as a camera (a visible light camera) taking a color video image of visible light, for example, when mounted to a lens device as disclosed in Japanese Patent Application Laid-Open No. 2005-4181. Accordingly, when a key signal is created by an infrared video image of the infrared light camera, the key signal appropriately distinguishes a main-object region of a color video image taken by the visible light camera from a background region. This enables to replace only a video image in the background region of the color video image taken by the visible light camera with another video image.